Curable resins which are reinforced with fibrous materials (composite materials) are commonly-used components in the automobile, wind turbine and aerospace industries. Such materials are light-weight, easy to mould and have good mechanical properties.
Curing of curable resins involves polymerisation and cross-linking reactions between the resin components, and is a generally exothermic process. During moulding of composite materials, curing is initiated by heating the composite material to above its cure onset temperature. So-called “latent” resins have cure onset temperatures which are above ambient temperature, so that they can be safely stored and handled without curing being initiated.
Curing of thick composite materials is particularly troublesome as the enthalpy of the curable resin builds up heat through the cure cycle. This heat has difficulty being dissipated out of the composite material during the reaction, meaning that high peak temperatures are reached during the cure.
For thick composite materials, such as glass or carbon fibre prepregs, the reaction energy of the prepreg is in the order of 100 J/g and this energy can give a temperature rise of approximately 100K on top of the temperature at cure onset temperature. For typical 120° C. curing prepregs the cure onset temperature is approximately 80° C. Such materials will see peak temperatures of up to 180° C. if the cure is done adiabatically. In thick composite materials, the exothermic energy produced by the curing reaction will raise the material core temperature to high levels, as the energy cannot be conducted to the surfaces to be dissipated.
Among other things, the high temperatures produced in composite materials due to the exothermic resin curing may cause thermal stress in moulding tools and thermal distortion in the moulded articles themselves.
There is therefore a need to provide curing of composite materials in a controlled manner, without generation of high peak temperatures.